Quote10.A - IC SlaveryEmphasis mine. I've not heard of a rules change saying otherwise.
As something of a specific corollary to these rules, please note that in-character enslavement or slave status of PCs does fall under mature content rules and the secret associations rules to a degree, because it is similar to torture/etc in that it's the sort of thing that forces players to react to it. In particular, this sort of thing is ICly illegal in Cormyr, and in most of the Dales as well, so it should not be brought up/must be kept quiet in those areas. Also, due to the implications inherent in such an event/story, existing characters cannot be made into slaves, even with OOC agreement, because this forces such an event on any number of other characters. As such, IC Slavery is limited to characters explicitly made for that purpose. If someone no longer wishes to play their formerly enslaved character, this requires retirement of that character, lest it provoke a giant PVP/drama situation that many unwilling players will inevitably get pulled into.
Please note that under 10.A, this is specifically limiting PC and PC interaction. A character may still have a background in slavery, and speak of it in general terms (Not explicit details), as long as the antagonist is an NPC. As said in the above rule - and that about backstories, trying to gather dislike or hatred towards a specific PC isn't kosher.